380 AGRICULTURAL ANALYSIS 



6.68 per cent, of nitrogen ; pyrogallic acid, 8.21 per cent. ; phenol, 

 13.62 per cent; benzaldehyde, 13.62 per cent.; phenyl salicylate, 

 13.72 per cent. It is interesting to note that phenyl salicylate 

 gave satisfactory results ; the oxidation, however, is not as rapid 

 as when salicylic acid is used. 



For ease in manipulation, rapidity of work, and accuracy of 

 results the salicylic acid method is to be recommended. 



330. The Official Kjeldahl Method for Nitric Nitrogen. As 

 has already been stated, the presence of certain organic com- 

 pounds rich in hydrocarbons permits the reduction of nitric 

 nitrogen to ammonia by combustion with sulfuric acid. Benzol, 

 phenol, and salicylic acid have all been used for this purpose. 

 The official chemists have adopted for their method the sali- 

 cylic acid process first proposed by Scovell. 24 



Besides the reagents and apparatus given under the kjeldahl 

 method there will be needed: 



1 i ) Zinc dust : This should be an impalpable powder ; granu- 

 lated zinc or zinc filings will not answer. 



(2) Sodium thiosulfate. 



(3) Commercial salicylic acid. 



It is found most convenient to prepare a solution of 33.3 grams 

 of salicylic acid in one liter of the strongest sulfuric acid, and 

 keep it for use rather than to mix it for each combustion. 



The Manipulation. Place from seven-tenths to three and five- 

 tenths grams of the substance to be analyzed in a kjeldahl di- 

 gesting flask, add 30 cubic centimeters of sulfuric acid containing 

 one gram of salicylic acid, and shake until thoroughly 

 mixed, then add five grams of crystallized sodium thiosulfate ; 

 or add to the substance 30 cubic centimeters of sulfuric acid 

 containing two grams of salicylic acid, then add gradually two 

 grams of zinc dust, shaking the contents of the flask at the same 

 time. Finally place the flask on the stand for holding the diges- 

 tion flasks, where it is heated over a low flame until all danger 

 from frothing has passed. The heat is then raised until the 

 acid boils briskly and the boiling continued until white fumes 

 no longer escape from the flask. This requires about five or 10 

 14 Division of Chemistry, Bulletin 16, 1887 : 51. 



